Piezoelectric transducer and method for producing same



Feb. 6, 1951 w L. CHERRY, JR 0,187

PIEZOELECTRIC TRANSDUCER AND METHOD FOR PRODUCING SAME Original FiledDec. 26, 1947 2 Sheets-Sheet l 5 F762 55 F/6f3 WALTER L. CHERRY JR.

INVENTOR.

HIS AGENT Feb. 6, 1951 CHERRY JR 2,540,187

W. L. PIEZOELECTRIC TRANSDUCER AND METHOD FOR PRODUCING SAME OriginalFiled Dec. 26, 1947 2 Sheets-Sheet 2 WALTER L. CHERRY JR. INVENTOR.

HIS AGENT Patented Feb. 6, 1951 PIEZOELECTRIC TRANSDUCER AND METHOD FORPRODUCING SAME Walter L. Cherry, Jr., Northbrook, Ill., assigno'r toZenith Radio Corporation, a corporation of minois Original applicationDecember 26, 194'! Serial No. 793,892. Divided and this application May2, 1950, Serial No. 159,813

3 Claims. 1

This application is a division of the copending application of RobertAdler, Serial No. 793,892, filed December 26, 1947, assigned to thepresent assignee, and relates to piezo-electric transducers and tomethods for producing such transducers. It is a primary object of theinvention to provide an improved artificial transducer having permanentpiezo-electric properties and to provide an improved method of producingsuch a transducer.

The use of polycrystalline aggregate such as barium titanate or bariumstrontium titanate, bonded with a ceramic binder, in the production ofartificial piezo-electric transducers is specifically disclosed andclaimed in the copending application of Walter L. Cherry, Jr., SerialNumber 770,163, filed August 22, 1947, for Piezo-.

Electric Transducers, and which application is assigned to the sameassignee as the present application.

Such artificial piezo-electric transducers as are specifically disclosedin the aforementioned copending Cherry application are particularlyuseful for high frequency applications. However, for optimum coupling,the direction of mechanical stress must be identical with that of thepiezo-electric axis and that of the alternating field; consequentlytransducers of this type are not readily applicable to audio frequencydevices. The minimum frequency at which such transducers may be operatedis determined by the maximum capacitance, or electrical compliance,commensurate with practical values of mechanical compliance. If themechanical compliance is made large enough to suit practicalrequirements by making the cross-sectional area small, the associatedcapacitance is too small for audio frequency applications. If, on theother hand, a practical value of capacitance is obtained by making thecross-sectional area large, the mechanical compliance becomesinconveniently small.

It is a particular object of this invention to provide an improvedartificial piezo-electric transducer which/is suitable for operation ataudio frequencies;

In accordance with the above-identified Adler application, it has beenfound that the capacitance or electrical compliance of the artificialtransducers may be increased while maintaining a practical value ofmechanical compliance by varying the direction of the piezo-electricaxis within a single-unit transducer. It is a further object of theinvention to provide a novel ceramic transducer, sensitive to bendingstress, which utilizes this principle.

Frequent applications of piezo-electric transducers in the audiofrequency range are made in phonograph pickups, microphones, and thelike. In such applications, it is necessary to provide a transducerwhich has permanent piezoelectric properties with respect to bendingstress. It is an important object of the present invention to provide anovel piezo-electric polycrystalline aggregate transducer, sensitive tobending stress, which is suitable for use as a phonograph pickup or thelike, and which is rugged, inexpensive, and durable.

In the specification and claims, the term direction, as applied to thepiezo-electric axis or to the polarizing field, includes the concept ofsense; therefore the phrase "piezo-electric axes having differentdirections includes a piezo-electric axes of opposite sense.

The term unidirectional, as applied to polarizing fields, is employedwith reference to time, and not with reference to space. Hence a"unidirectional polarizing field is one which is produced by aunidirectional potential difference.

The terminology polycrystalline aggregate is employed to connote aunitary structure comprising a large number of minute crystals. The termceramic necessarly implies such a structure.

The features of the invention which are believed to be novel are setforth particularly in the appended claims. The invention may morereadily be understood, however, by reference to the folowing descriptiontaken in connection with the accompanying drawings, in which likereference numerals indicate like elements, and in which:

Figure 1 is a schematic representation of a theoretical means forattaining the objects set forth above.

Figure 2 is a schematic representation of a practical method forapproximating the theoretical optimum condition shown in Figure 1.

Figure 3 is an enlarged view of a section of Figure 2 showing theelectrostatic fiux distribution.

Figure 4 is a schematic representation, partly in section, of a ceramicelement sensitive to bending stress.

Figures 5 and 6 are schematic representations of an embodimentconstructed in accordance with the invention.

Figures 7-9 are schematic representations of another type of ceramictransducer sensitive to bending stress.

Figure is a perspective view of a physical embodiment of the invention.

Referring now to Figure 1, there is shown schematically a slab ofpiezo-electric material, the direction of the piezo-electric axis beingassumed to be longitudinal. It is seen that the mechanical compliance inthe longitudinal direction may well be high enough to suit practicalrequirements, since the cross-sectional area is small relative to thelength, but the capacitance between electrodes located on thesmall'surfaces 2i and 22 separated by the full length of the piece 20 isinconveniently small. In order to increase the capacitance or electricalcompliance, it is assumed that slab 28 is theoretically divided into nslices 23-42 of equal length; the dotted lines indicate any desirednumber of intermediate slices. The capacitance between the imaginarytransverse surfaces of each slice is therefore n times that between thetwo end surfaces 2! and 22 of slab 28, while the mechanical compliancebetween surfaces 2i and 22 remains unchanged. Furthermore, byinterconnecting alternate transverse faces of the imaginary slices23-32, as shown, and by connecting each set of alternate transversefaces to one of a pair of terminals 88 and 24, the total capacitancebetween surfaces 2| and 22 is made equivalent to n times the capacitancebetween the end faces of an individual imaginary slice 23; consequentlythe effective capacitance between surfaces 2! and 22 is increased by afactor of n.

Ideally such a-condition may be accomplished by polarizing adjacentslices 28-32 along the same axis but in opposite directions. Such acondition is shown schematically by the arrows which indicate thedirection of polarization, and hence the direction of the piezo-electrlcaxis, in each of the imaginary slices.

It has been found that permanent pleas-electric properties may beinduced in certain polycrystalline aggregates, and furthermore, that thedirection of the piezo-electric axis induced in such aggregates isidentical to that of the electrostatic flux set up by the polarizingfields. Consequently, such a condition as shown schematically in Figure1 might be accomplished if suitable unidirectional polarizing fieldscould be set up between the transverse faces of each of the imaginaryslices 23-32. Obviously, such a situation is impractical, since noelectrodes exist on the imaginary transverse surfaces between theslices.

As a practical approach to the optimum condition shown in Figure 1,there is shown in Figure 2 a slab of suitable polycrystalline aggregate,such as a ceramic comprisi barium titanate or barium strontium titanatemixed with a small amount of a glass forming oxide and fired tovitrification in accordance with the aforementioned copending Cherryapplication, on opposite faces 4i and 42 of which have been disposed aplurality of electrical terminals or electrodes 43-44. For purpose ofillustration, twelve terminals have been shown: however, other numbersof terminals may be employed. Alternate pairs of opposite terminals 43,ll, l1, 48, 5|, 52 and 45, 48, 49, 50, 53, 54 are interconnected andbrought out to a pair of input terminals 55 and 56 respectively. when aunidirectional potential diflerence is applied between input terminals55 and 58, adjacent portions of slab III are polarized in substantiallyopposite directions.

In order more fully to show and explain such polarization, there isshown in Figure 3 a detail view of a portion of slab 40 of Figure 2. Theelectrostatic flux distribution, and hence the direction ofpolarization, is assumed, for the purposes of explanation, to have asense from to If then, it is assumed that input terminal 55 is thepositive terminal and input terminal 58 is the negative terminal, theelectrostatic field, and hence the direction of the piezo-electric axis,differs abruptly from portion to portion as schematically shown inFigure 3.

It is seen that the embodiment shown and described in conjunction withFigures 2 and 3 is only an approximation of the ideal shown anddescribed in conjunction with Figure 1. Since those portions of the slab40 of ceramic material which lie between opposite pairs of terminals, l1and 48, for example, carry little or no electrostatic flux, theseportions may be regarded as being waste portions." In order to effect acompromise between the desired effective increase in electricalcompliance and a minimum of "waste material, it has been found that thewidth of the individual terminals 43-54 should be of the order of thethickness of the slab l8, and the distance between successive terminals,41 and 49, for example, should be of the order of twice the thickness ofthe slab 40. These proportions are the result of practical experimentand are intended in no sense to be construed as limitations, as otherproportions may be used with varying degrees of efficiency.

In certain audio frequency applications, it is 'desirable to employ apiezo-electric element which is sensitive to bending. Two elements, eachformed in accordance with the method shown and described in connectionwith Figure 2, may be fastened together, by cement or in some othersuitable manner, to provide such a dimorphic element. Such aconfiguration is shown in Figure 4, wherein a pair of elements 60 and 6|are fastened together to form a composite body 82. In operation oneelement 60 is instantaneously subjected to tensile stress while theother element 8! is instantaneously subjected to compressive stress, orvice versa. Therefore, care must be taken that elements 60 and iii areassembled and electrically connected in such a manner that the usefuloutputs of the individual elements are additive in the compositetransducer 62.

While a transducer having permanent piezoelectric properties withrespect to bending stress may be produced in the manner shown anddescribed in conjunction with Figure 4, a simplified transducer of thistype may be provided. In

accordance with the invention, an element sensitive to bending stressmay be provided by properly applying polarizing fields to a singleunitary polycrystalline aggregate body. Such an element is shown inFigure 5, wherein a unitary slab 18 of barium titanate or other suitableaggregate is provided with a plurality of electrical terminals orelectrodes ll-82 similarly disposed along opposite faces 83 and 84.Alternate terminals H, 15, and 19 on face 83 and alternate terminals 14,I8, and 82 on face 84 are interconnected and brought out to an inputterminal 85, and the remainder of the terminals 12, 13, I8, l1,

10 80, and 8| are interconnected and brought out to a second inputterminal 86.

By applying a unidirectional potential differ-i ence between inputterminals 85 and 86, unidirectional polarizing fields are appliedbetween 1 successive terminals, II and 13, for example, on

s each face, 04 of slab I0. Furthermore, each terminal (H, for example)is oppositely polarized from the terminal (12, for example) which isdirectly opposite therefrom, and the polarizing fields between oppositepairs'of successive terminals ill, I and II, I4 for example) areopposite in sense. By these means, the direction or sense of thepiezo-electric axis is made to differ abruptly from portion to portionthroughout the single piece II in a manner similar to the piezoelectricaxis distribution in a composite dimorphlc'element such as that shown inFigure 4.

After maintaining the polarizing fields for a suiiicient period of timeat least to approach saturation of the piezo-electric eifect, suchfields are removed, and the connections of terminals I I-fl are changedto insure additive outputs in response to bending. Slab I0 withterminals II-02 reconnected for proper output at terminals 00 and 08 isshown in Figure 6, in which alternate pairs of opposite terminals II,I2, I0, I0, 10, and 00, and I3, I4, TI, I0, 8|, and 02 areinterconnected. Such an interconnection is desirable in order tominimize the shunt capacitance between output terminals 85 and 00 whicheffectively reduces the useful output of such a reducing any shuntcapacitance across the output terminals. In this embodiment, a pair ofpolycrystalline aggregate bodies 90 and 0i are fastened together bycementing means 92 of low dielectric constant thereby to form acomposite body. Cementing means 02 may comprise a layer of cementingmaterial of low dielectric constant, although the desired results may beachieved by glazing slabs 00 and 0| and firing with the glazed sides incontact, by firing a "sandwich of three ceramic slabs, or by othersuitable means. Opposite faces of the composite body are provided with aplurality of similarly disposed electrical terminals 93-98 and 99-404,alternate pairs of opposite terminals 83, 99, 95, MI, 91, I03, and 94,I00, 96, I02, 08, I04 being interconnected and brought out to a pair ofinput terminals I05 and I06 respectively. After polarizing in the usualmanner, the terminal connections on one face of the composite body arereversed to insure additive outputs in response to bending, as shown inFigure 8, and terminals I05 and I06 become output terminals. Althoughopposite terminals (93 and 90, for example) have opposite polaritiesassociated therewith, the thin layer of cementing material 92 so reducesthe effective capacitance shunting the output that satisfactoryoperation is insured. The embodiment of Figures 7 and 8 is specificallydisclosed and claimed in the copending application of Alexander Ellett,Serial No. 159,620, filed concurrently herewith, for Pie'zo-EiectricTransducer and Method for Producing Same, and assigned to the presentassignee.

While the embodiment shown in Figures '7 and 8 is formed by cementingbodies 90 and SI together before polarization, it is to be understoodthat the individual bodies 00 and SI may be polarized separately andthen firmly united in such manner as to insure additive outputs inresponse to bending. Such a method is shown schematically in Figure 9,in which body .90 is separately polarized. Alternate terminals 93, 95,91, and 94, 06, 90 are connected to respective input terminals I00 andI06. After polarization, two such bodies are cemented or otherwisefastened together to produce a composite body such as that shown inFigure 8.

It is also to be understood that, although pairs of terminals 03-404have been shown connected in parallel, pairs of terminals may beconnected In series in certain applications in which such connection isdesirable.

As a variant of this embodiment, two slabs of polycrystalline aggregatesuch as slab 00 in Figure 9 may be provided each with electricalterminals differently disposed in such a way that, after polarization,such slabs may be fastened together in a manner similar to that shownand described in conjunction with Figure 4. In this way, objectionableshunt capacitance between the output terminals may be avoided withoutthe use of low dielectric constant cementing means.

As a further variant, the composite body shown in Figures 7 and 8 maybepolarized with the terminals 93 I04 connected as in Figure 8, in whichcase the terminals 83-I04 are reconnected as shown in Figure 7 to insureadditive outputs. In this manner, the shunt capacitance across theoutput is minimized by interconnecting opposite terminals.

There is shownin Figure 10 a phonograph pickup IIO constructed inaccordance with the invention. The terminals III and H2 are shown in theform of a pairof intermeshing combs for efilcient polarization, and maybe of silver paint or other suitable material applied by silk screen,vacuum evaporation, or other suitable process. In a practicalapplication, the terminals applied on the back side (not shown) ofelement I I0 may be made in theform of a mirror image of those appliedto the front side in order to minimize the undesired capacitanceshunting the output. One end H3 of transducer 0 is firmly clamped in abracket I I4, and the other end I I5 is provided with a rigid extensionH0 so constructed and arranged that the edges I I1 and I I0 oftransducer H0 and extension H0 all converge to a point H0. Lateralmotionat point I I9, which may be translated from the undulations of a recorddisk by means of a stylus II9' secured to extension H6 at point H9, thenresults in corresponding electrical output between terminals III and.H2; by making transducer IIO trapezoidal in shape, the conditions of auniform-stress beam are approached.

While there .has been shown and described a certain preferred embodimentof the invention, it will be understood that numerous variations andmodifications may be made, and it is contemplated, in the appendedclaims, to cover all such variations and modifications as fall withinthe true spirit and scope of the invention.

I claim:

1. The process of producing a permanently piezo-electric ceramic bodysensitive to bending stress, said process comprising similarly disposingalong each of twoopposite faces of a solid polycrystalline aggregate aplurality of parallel electrodes, applying unidirectional polarizingfields having substantially-opposite directions between successive pairsof electrodes on each of said faces, the direction of the polarizingfields between successive pairs of electrodes on one of 2. A ceramicelement having permanent piezoelectric properties, said elementincluding a solid polycrystalline aggregate and a plurality ofelectrodes similarly disposed along each of two opposite faces of saidaggregate, the direction of the piezo-electric axis being substantiallyopposite between successive pairs of electrodes on each of said faces,and the direction of the piezo-electric axis between successive pairs ofelectrodes on one of said faces being opposite to the direction of thepiezo-electric axis between corresponding pairs of electrodes on theother of said faces.

3. A ceramic element having permanent piezoelectric properties, saidelement including a solid polycrystalline aggregate, said aggregatecomprising individual crystals of barium titanate bonded together with aceramic binder, and a plurality of electrodes disposed along each of twoopposite faces of said aggregate, the direction of the piezoelectricaxis being substantially opposite between successive pairs of electrodeson each of said faces, and the direction of the piezo-electric axisbetween successive pairs of electrodes on one of said faces beingopposite to the direction of the piezo-electric axis betweencorresponding pairs of electrodes on the other of said faces.

- WALTER L. CHERRY, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS It! Number Name Date 2,281,778 Mason May 5, 19422,486,560 Gray Nov. 1, 1949

